1. Field of the Invention
The present invention relates to an explosive pipe cutting device for severing pipe, such as drill pipe and tubing used in oil wells, natural gas wells and other types of wells.
2. Related Art
It is often desirable to retrieve pipe, tubing and the like (below referred to as "pipe") from deep within a well, such as an oil well which is being closed or abandoned. Such pipe may extend for many tens of thousands of feet into the well and, in some cases, is made of expensive, high strength steel. Consequently, the ability to retrieve and reuse such pipe provides a very considerable cost savings as well as recycling a non-renewable resource. Retrieval is accomplished by cutting the pipe deep below the surface with an explosive shaped charge and withdrawing for re-use the portion of the pipe above the point at which it was cut. The amount of savings to be attained increases with increasing depth of the well. As the depth within the well increases, however, there is a concomitant increase in both (1) the pressure and temperature at which the explosive pipe cutting device must function and (2) the length of pipe which must be navigated by the pipe cutting device as it is lowered into the well to the point at which the pipe is to be severed. Typical explosive pipe cutting devices comprise a housing within which is contained an explosive shaped charge having in the known manner a metallic liner on their concave surfaces. In addition to the shaped charge, the housing typically contains a booster explosive to reliably initiate the shaped charge, an initiation device to reliably initiate the booster explosive, and an end plate serving to securely retain the components within the metal housing. The metal housing serves to protect and enclose the shaped charge and other components. The explosive cutting device is connected to a "wireline string" which is utilized to lower the cutting device to the desired depth, which may be ten thousand feet or more, at which depth the pipe is to be cut. The wireline string typically comprises a braided steel outer jacket which provides mechanical strength and has an electrically insulative core through which wire conductors pass to transmit, in response to a signal generated at the surface, electrical energy to a detonating fuze contained within the housing and associated with the booster charge. Electric current passed through these conductors initiates the detonating fuze, which detonates the booster charge, which in turn detonates the shaped charge to attain the explosive cutting effect.
The housings of known explosive pipe cutting devices are usually made of hardened steel, are of circular cylindrical configuration, and terminate in a flat lower end or nose portion. For example, a conventional housing might be machined from a solid steel circular bar into a cup shape with the closed (nose) end of the cup in the configuration of a flat disk. Such cylindrical shaped housings are relatively inefficient in resisting the pressure encountered in deep wells, and therefore require a large wall thickness for a given level of pressure, especially of the nose end, which is made thicker than the walls of the circular cylinder. The large wall thickness adds to the amount of hardened steel debris deposited in the well bore upon detonation of the shaped charge. Further, the flat nose housings are difficult to maneuver around obstructions in the well.
Typical shaped charges of known construction for use in severing pipes are of toroidal configuration with a metal-lined, circumferential concave opening extending about the outer periphery of the toroidal structure. As is well-known to those skilled in the art, the metal liner increases the mass of the high velocity explosive jet generated by the shaped charge. The toroidal configuration is attained by positioning two annular half-charges together so that each annular half-charge provides one-half of the finished toroidal shaped charge, the two half-charges being symmetrical about a plane passed through the apex of the concave, circumferential opening perpendicularly of the longitudinal axis of the toroidal shaped charge. Prior art toroidal shaped charges utilize an annular metallic ring on each half-charge which, when the two halves are joined together, define a metal liner having a V-shaped cross section and lining the concave circumferential opening of the assembled shaped charge. It would be advantageous to enhance the penetrating power of the shaped charge by improving the design of the liner to increase the metal mass at the apex of the liner.
Another problem in the art is to properly align the halves of the shaped charge because the alignment thereof determines the symmetry of the two half-charges which is critical to enhancing the penetrating power of the explosive jet and hence its reliability in effecting a complete break in the pipe.